Instead of spending $10-20 on a commercial wall shelf why not make your own from a strip of crown molding. You'll also need a board for the top of the shelf and whatever sandpaper and paint or varnish you need to get it looking the way you want.

Instructables user hoodwoodwork recommends measuring how wide you want the shelf, cutting the sides of the molding to match, and nailing the molding piece to the shelf top. You may want to create another layer of molding as in the photo above—once you're happy sand and paint or varnish the shelf.